The present invention relates to an improved sample cell for use with X-ray fluorescence analytical devices.
Sample cells that are used for positioning a sample within an X-ray fluorescence analyzer typically employ a telescopically interfitting cup and ring combination for capturing there between and thereby supporting a radiation-permeable membrane through which the sample is analyzed. After a sample is placed in the cup and the radiation-permeable membrane is positioned across the mouth of the cup, the ring is forced down over the membrane and down along the exterior surface of the cup. The interfitting components serve to capture the membrane there between and pull it taut across the mouth of the cup. Upon inversion of the cup, the sample becomes supported on the flat, wrinkle-free membrane surface on which it is positioned over the X-ray source for analysis. Such sample cells are intended for a single use and are disposed of after the analysis is completed. The cup and ring components are therefore typically molded from a synthetic plastic such as nylon, polyethylene, polypropylene or polyester.
In order to cause the membrane to be tightly and uniformly stretched across the mouth of the cup and to then be maintained in such condition, it had been found to be necessary for the cup and the ring to be very tightly interfitting. The use of such tight tolerances were, however, found to be problematic in that the membrane was thereby prone to bind over the lip of the cup and to then tear or pucker as the ring was pushed into place. While the application of a lubricant reduced the risk of damaging the membrane, considerable time and effort were required in order to properly apply the lubricant so as to avoid contact with the interior of the cup and to thereby avoid contaminating the sample.
Sample cell configurations were subsequently developed to address these shortcomings and included the incorporation of a locking rib and recess combination. A circumferential rib formed on the interior surface of the ring was positioned and dimensioned to cooperate with a circumferential recess formed on the exterior surface of the cup. The inherent flexibility of the plastic used in the construction of the two components allowed the rib to snap into place within the recess as the ring was pushed into place about the cup. This not only served to lock the ring into position on the cup but to also positively hold the membrane there between. A lubricant was additionally incorporated in the plastic to reduce the coefficient of friction between the cooperating components. It was found that the use of silicone oil in combination with virgin polyethylene provided the desired result without the risk of contaminating the sample. The use of the locking mechanism as well as lubricant-containing plastics ensured that a taut and wrinkle-free surface could easily and consistently be achieved and maintained without damaging the membrane during the assembly process. Such device is described in U.S. Pat. No. 4,448,311 which is incorporated herein by reference.
The development of more sensitive X-ray fluorescence analyzing equipment has necessitated the reconfiguration of sample cells. Previously used sample cells had been configured such that the surface of the membrane is positioned outwardly beyond the edge of the fully seated ring component. As a result, the placement of the cell onto a surface while in its inverted state would necessarily cause the membrane to contact such surface and risk the transfer of contamination therefrom. The presence of even minute traces of contamination on the exterior surface of the membrane could compromise the analytical results with the use of the more sensitive analyzing equipment. While the risk of contamination could conceivably be avoided if the sample cell were to be inverted only for final placement over the X-ray source, certain automated analyzers require the cell to first be placed onto a receiving surface while in its inverted state from which it is then automatically transferred to the testing site. The risk of contamination can thereby not readily be avoided.
In order to address this problem, analyzers have been developed that require the membrane of the sample cell to be spaced up off of a flat supporting surface. The spacing of the membrane up off of such a supporting surface has typically been achieved with an increase in the ring height such that its edge extends beyond the edge of the cup and hence the membrane surface when in its fully seated position. In order to allow the membrane to then come into contact with the aperture surface of the analyzer device so as to eliminate any air gap therebetween, the aperture of the device has formed thereabout a receiving groove which is dimensioned to accommodate the protruding edge of the inverted sample cell. Once a selected cell is brought into position above the analyzer""s aperture by, for example, an automated sample handling device, it is lowered into place in an effort to allow the protruding lip to drop into the recessed groove. It has however been found that even a very slight misalignment of the cell vis-a-vis the groove can cause difficulties. Should one side of the cell""s edge hang up along one edge of the groove while the opposite side drops into the groove, the skewed orientation of the cell will fail to eliminate the air gap between the cell membrane and the aperture surface while the bottom surface of the sample will be angled relative to the X-ray source and sensor. A slightly greater misalignment of the cell vis-a-vis the analyzer aperture could cause the sample cell to remain balanced on the edge of the receiving groove and although such orientation would maintain the bottom of the sample properly angled relative to the X-ray source and sensor, an undesirable air gap will nonetheless remain between the sample cell membrane and the aperture surface. In either scenario, operator intervention would be required in order to rectify the misalignment.
An improved sample cell configuration is therefore needed which can more readily accommodate a slight misalignment of the position of the sample cell vis-a-vis the receiving groove of the analyzer device. The configuration of the sample cell should facilitate its complete receipt in the groove despite small radial or angular misalignments and preclude the assumption of orientations that would require operator intervention.
The present invention provides a sample cell having a configuration that will more readily drop into the receiving groove that is formed about an analyzer aperture despite slight misalignment of the sample cell relative to the receiving groove. Thus, such sample cell will be less inclined to assume a skewed or an inappropriately offset orientation when deposited onto the aperture surface by an automated sample cell handler such as a turntable or carousel device.
More particularly, the sample cell of the present invention employs a ring element that has a uniquely configured protruding lip on its distal end. The protruding nature of the lip allows the sample cell to be supported on a flat surface so as to automatically maintain the membrane out of contact with such surface and thereby preclude the transfer of contamination therefrom. Additionally, it has been found that the spacing of the membrane up off of a supporting surface serves to reduce the risk of damage to the membrane such as could be caused by a sharp edge or burr that may be present on such surface. The lip is radiused in a manner that has been found to enable the protruding portion of the sample cell to more readily drop into place within the receiving groove that is formed about the aperture surface of the analyzer device. When viewed in cross section, both the exterior edge of the lip as well as the interior edge of the lip are radiused. Moreover, the radius on the exterior edge is formed so as to define a convex surface while the radius on the interior edge is formed with a reverse radius so as to define a concave surface. The smooth exterior surface of the ring along with the incorporation of a lubricant in the material from which the ring is formed further enhances the ability of the sample cell to drop into the analyzer""s receiving groove.
The ring element of the sample cell of the present invention additionally has a radially extending flange extending from its proximal end. Such flange enables an automated sample handler such as a turntable or carousel device to pick up the device and move it to and from the analyzer aperture.